The invention relates to an arrangement for cancelling an echo in a mobile system, an echo canceller in a mobile system and a transcoder.
Referring to FIG. 1, in current telephone systems, a telephone exchange and a speech processing unit are interconnected via a logical and/or a physical interface. Speech is transmitted over this interface in a packed form, i.e. companded, for example as PCM samples coded in accordance with the A or the xcexc law. Generally, at an interface of this type, the bit rate for a speech channel is 64 kbits/s. A well known example of a system of this type is the interface between a mobile exchange MSC in the GSM system and a transcoder unit TRAU (Transcoder and Rate Adaptation Unit), the interface being referred to as an A interface. When a call is established in a mobile system between two mobile stations, for example MS1 and MS2, speech originating from the mobile station MS1 is coded twice, first in the mobile station MS1 and then in a transcoder unit TRAU2. This situation is referred to as tandem coding. Tandem coding degrades the speech quality.
Various solutions have been presented for preventing tandem coding. Such a mechanism is disclosed in reference 1. Within the scope of the present application, preventing tandem coding is referred to as TFO (Tandem Free Operation). If the TFO is active, speech is transmitted over the A interface as speech parameters and not as samples in accordance with the A or the xcexc law. According to reference 1, it is also possible to transmit both representations of speech simultaneously in parallel in the speech channel. In FIG. 1, a conductor 11 drawn with a solid line illustrates speech coded into PCM samples, and a conductor 12 drawn with a dashed line illustrates speech coded into a parametric format by means of a speech decoder in the mobile system.
Mobile stations bring about an acoustic echo audible to the other party. FIG. 1 presents a situation of this type wherein speech spoken to a mobile station MS1 is encoded by means of a speech encoder in the mobile station. The encoded speech propagates to a second mobile station MS2, where the encoded speech is decoded and delivered to a loudspeaker in the mobile station. Part of the speech loops back to a mobile station MS1 user as an echo via a microphone in the mobile station MS2. Normally, the echo should be cancelled in the mobile station, but because of a deficient implementation of echo cancellation, situations occur where a low-level but yet disturbing acoustic echo may be transmitted. This residual echo can be cancelled by means of an acoustic echo canceller on the mobile telephone network side. Early echo cancellers prevented the echo from returning to the speaker by simply cutting off a signal path when it was detected that the echo must be cancelled. However, the speaker may find a total silence disturbing as he may think that the connection is broken. The latest echo cancellers therefore send so called comfort noise (CN) to the speaker, the comfort noise being generated by taking samples from the background noise of a B subscriber. The echo canceller AEC can be a separate network element. Alternatively, an echo cancellation operation can be located or integrated in connection with another network element. For the sake of simplicity, both the xe2x80x9coldxe2x80x9d type of silence and the xe2x80x9cnewxe2x80x9d type of actual comfort noise are hereinafter referred to as comfort noise.
When the TFO operation is active, the speech parameters are transmitted between the MS1 and the MS2 directly without coding them into A-law or xcexc-law samples. In FIG. 1, this operation is illustrated by a conductor 12. If at least one of the mobile stations generates an acoustic echo, the echo is transmitted to the other mobile station unless the acoustic echo canceller (AEC) is used.
Referring to FIG. 2, an alternative is to develop comfort noise directly in a parametric format, whereby original speech samples are replaced with the parametric presentations illustrating the comfort noise. A solution of this type is presented in reference 2. A problem with this solution is that generating noise in this manner must be carried out in a different manner for each speech coding method. In that case, a comfort noise analyzing operation in the mobile station, or, alternatively, separate encoding and comfort noise analyzing operations in the echo canceller are needed for processing at a parametric level.
Referring now to FIG. 3, a known mechanism is to prevent the TFO operation when the AEC is active. The mechanism in FIG. 3 operates as described below. The AEC monitors downlink speech activity (MSC2xe2x86x92MS2). When the AEC detects downlink speech activity, it replaces an uplink speech signal (MS2xe2x86x92MSC2) by comfort noise. The comfort noise is generated as PCM samples. At least one problem with this mechanism is that tandem free operation must be renegotiated between the transcoder units TRAU always after the use of an AEC. This technique dramatically degrades the speech quality.
Yet another known technique is not to use the AEC if the TFO is active (FIG. 1, conductor 12). In this case, a disturbing acoustic echo is clearly audible.
An object of the invention is thus to provide an apparatus for solving the above described problems. A particular object of the invention is to allow the operation of an echo canceller without at the same time disturbing tandem coding cancellation. The objects of the invention are achieved by apparatuses which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the fact that speech parameters transmitted in a mobile telephone network must be changed in some way in order to cancel an echo. At the same time, tandem free operation is desired to be maintained active in order to maintain the speech quality as good as possible.
A potential solution to this problem is presented in FIG. 4. This solution is based on the fact that the speech coding method employed in TFO is used for coding comfort noise into a parametric format and for replacing the original speech parameters by the parameter presentations illustrating the comfort noise. In this embodiment, an additional encoding operation is needed for the echo canceller.
A preferred embodiment of the invention presented in FIG. 5 is based on the fact that a comfort noise generation operation that generates comfort noise as PCM samples (i.e. as A-law or xcexc-law samples) is located in an echo canceller. In a call between two mobile stations (or in general in a call whose all parties are capable of the TFO operation), the tandem free operation (TFO) is active, i.e. speech is transmitted in a parametric format between transcoders. In order to achieve this, it is also assumed that the PCM samples (i.e. the A/xcexc-law samples as presented in reference 1) are transmitted simultaneously with the parameters. In addition, it is assumed that some kind of frame-form signalling information is also transmitted between the transcoders.
It is an advantage of the echo cancellation of the invention that it also cancels the acoustic echo during the TFO operation. It is a particular advantage of the preferred embodiment of the invention that not much additional processing is required. Another advantage is that the invention enables the use of the same comfort noise generation means irrespective of the codecs used in the mobile system.